1. Field of the Invention
The present invention relates to a valve gear of an internal combustion engine (hereinafter referred to as engine).
2. Description of the Related Art
To realize engine-output characteristics optimum for operating regions, there have been proposed a variety of engines adapted to switch valve-opening periods and lift amounts of intake and exhaust valves, etc. (refer to Japanese provisional patent publication no. 2001-41017, for example).
The engine disclosed in this publication is provided with a driven rocker arm supported by a rocker shaft so as to be rocked by a first cam for low speed to thereby drive the intake valve to open and close, and a drive rocker arm supported by the rocker shaft at a location adjacent to the driven rocker arm so as to be rocked by a second cam for high speed. The driven rocker arm is formed with a cylinder in which a piston is accommodated for sliding motion when supplied with oil pressure, and the drive rocker arm is formed with an engaging projection adapted to be engaged with the piston when the drive rocker arm is rocked.
In a low rotation region of the engine for example, the piston of the driven rocker arm is switched to a lower position where the engaging projection of the drive rocker arm runs at idle, so that the intake valve is driven to open and close by the driven rocker arm along the shape of the first cam. In a high rotation region of the engine, on the other hand, the piston of the driven rocker arm is switched to an upper position where the engaging projection of the drive rocker arm acts to press the piston, so that the driven rocker arm is rocked in conjunction with the drive rocker arm, whereby the intake valve is driven to open and close along the shape of the second cam.
As described above, the engine disclosed in Japanese provisional patent publication No. 2001-41017 is designed to make the switching between the two operating conditions in each of which the intake valve is driven to open and close by a corresponding one of the first and second cams. There is the demand such as for example that the engine of this type be provided with not only the just-mentioned function but also a cylinder suspending function of suspending the operation of a particular cylinder.
To meet such demand, it is considered that a further drive rocker arm may be provided so as to be rocked by the first cam as in the case of the drive rocker arm associated with the second cam, and a pair of pistons may be provided in the driven rocker arm to be slidable for engagement with engaging projections of these drive rocker arms. In such valve gear, depending on the sliding positions of the pistons, the driven rocker arm is rocked in conjunction with either one of the drive rocker arms to thereby drive the intake valve to open and close according to the shape of a corresponding one of the cams. When the pistons assume their sliding positions where the engaging projections of the drive rocker arms run at idle, the driven rocker arm maintains the intake valve closed, whereby a cylinder suspending operation is performed.
To be noted, the switching of the piston position in the driven rocker arm is made utilizing oil that is discharged from a lubricant oil pump of the engine. In a low rotation region of the engine, the rotation speed of the oil pump is low and hence an adequate amount of discharge cannot be expected. For this reason, in a low rotation region where the piston position is switched in the driven rocker arm on the side of the first low-speed cam, the piston's sliding motion is slow in speed. This poses a problem that the engine operating condition cannot be switched promptly.
Meanwhile, an upper portion of the piston of the driven rocker arm, which, when pressed by the engaging projection of the drive rocker arm, causes the entire driven rocker arm to be rocked against the inertial mass, is required to have a higher rigidity as compared to the lower portion of the piston that mainly receives an oil pressure alone. In this regard, the engine disclosed in Japanese provisional patent publication No. 2001-41017 has a piston that is made in its entirety of a material that satisfies the characteristic required for the upper portion of the piston. As a result, each individual piston becomes inevitably high in fabrication cost, which affects the fabrication cost of the entire valve gear requiring the provision of the piston for every drive rocker arm of each cylinder.